Experimental evidence supports the conclusion that in certain animal models, specific subpopulations of lymphocytes may normally regulate hemopoiesis by directly influencing the proliferation of hemopoietic stem cells. In man, however, the physiologic effects of lymphocytes upon hemic precursor cells are not well defined. We intend to study in detail the effects of isolated subsets of human lymphocytes upon the proliferation of committed granulocytic-monocytic progenitor cells (CFU-c) in vitro. Peripheral blood "null" cells -- free of contamination by monocytes, B-cells and T-cells -- are highly enriched in hemopoietic progenitor cells in man and will be used as a source of CFU-c in all experiments. Isolated B-lymphocytes, unfractionated T-lymphocytes, TH2 (plus) T-cells, or TH2 (minus) T-cells will be simultaneously harvested from healthy volunteers by standard cell separative techniques. In some experiments, isolated lymphocyte subfractions will be directly mixed with autologous "null" cells and incubated in CFU-c cultures in the presence and absense of appropriate soluble mitogens to determine whether these cells are capable of directly inhibiting or stimulating CFU-c growth. As an adjunct to these studies, similar experiments will be carried out using cell fractions isolated from multiply transfused patients. In other experiments, isolated subpopulations of lymphocytes will be incubated in short-term liquid suspension cultures in the presence and absence of monocytes and soluble mitogens, and media conditioned by these cells will be assayed for the presence of granulocyte-monocyte colony stimulating activity (CSA).